The present invention concerns separation of liquids and particulate solids from a stream of gas, particularly in a production process of oil and gas. More specifically the present invention concerns a separator for separation of material compositions of gas, liquids and particulate solids with a separator comprising axial flow demisting cyclones for the final removal of liquids and particulate solids before the gas is discharges from the separator. The axial flow demisting cyclones has a drainage system ensuring that liquids and sand will not be able to accumulate any where in the axial flow demisting cyclones or drainage channels but will instead be directed along with any liquid present into the separator liquid compartment.
In the production of oil and gas from a subterranean or subsea reservoir the well production flow almost always will contain water and a little sand along with oil and gas. For this reason a plant for receiving and separating the individual phases are arranged. The separation is conducted in different steps, a “coarse” separation utilizing only gravity and “fine” separation utilizing centrifugal forces and moment of inertia along with gravity. The separation is conducted in large separators arranged either horizontally or vertically.
In separators several steps of liquid separation can be performed. Firstly the gas is fed through an inlet conduit which for vertical separators can be about at the middle of the separator's vertical extension. At the inlet a baffle plate or a vane diffuser is typically arranged to distribute the inlet flow over the separator cross-section. Already at this stage the largest drops are separated out and fall into a liquid reservoir in the lower part of the separator.
The gas flow moves upwards in what can be denoted a calm zone or deposition zone where further drops of liquid are caused by gravity to fall down to the liquid surface below, possibly after having been deposited on the separator wall and drained along the wall surface.
Close to the outlet at the top of the separator the gas is brought to flow through a number of parallel demisting cyclones or other demisting equipment of prior art technology for removal of drops and particulate solids which are not separated from the gas flow by gravity. From the demisting cyclones the liquid and particulate solids is directed into a manifold system and directed further down into the liquid reservoir below through one or more drainage pipes the lower end of which are positioned below the liquid surface of the liquid reservoir at the lower end of the separator. Norwegian patent 320 351 shows examples of embodiments of such separators and in this publication is also explained the total requirement for having the lower end of the drainage pipes arranged below the liquid surface of the liquid reservoir at the separator lower end. At high pressure drops from the cyclone inlet to the cyclone drainage chamber there will be a corresponding sectional force in the drainage pipe which results in a higher liquid level in the drainage pipe than in the reservoir. If the pressure drop becomes too high, liquid is sucked up through the drainage pipe instead of being drained downwards through the pipe. If (the lower end of) the drainage pipe is not immersed in the liquid phase the sub-pressure in the cyclone drainage chamber will result in a gas flow upwards in the drainage pipe that counteracts the drainage and partly or completely prevents separation.
The arrangement of demisting cyclones here described is characterized in that the liquid separated out is led back to the same chamber and thus the same pressure as the cyclones are being fed from. The arrangement with the immersed drainage pipes which have a function of a liquid seal is decisive for the functionality of the arrangement.
Two main types of demisting cyclones are known. One type is characterized by a change of flow direction of the gas within the demisting cyclone and is referred to as reverse flow demisting cyclone while another has only one direction of flow and s referred to as an axial flow demisting cyclone or simply axial cyclone. Both these types are discussed in NO 315188. While the first type is characterized by an undesired high pressure drop the axial flow demisting cyclone has a lower pressure drop and is better suited for the final demisting in separators.
On the other hand reverse flow demisting cyclones are better suited for taking care of particulate material because the particles will be conveyed vertically downwards, i.e. in the direction aided by gravity, along with separated liquids. Solid particulates and liquid will thus not be able to accumulate in the cyclone or in the drainage system.
This is not the case for axial flow demisting cyclones where liquid and particulate solids will be able to accumulate within the drainage system of the cyclone arrangement unless the arrangement is made according to the resent invention. A number of axial flow demisting cyclones are normally arranged in a cassette, typically four cyclones per cassette. The cassettes can be arranged so that the cyclones have their longitudinal direction either in the horizontal or in a vertical plane. Each cassette has a drainage conduit which is connected to a common, horizontally arranged manifold which again is communicating with one or more vertically oriented drainage pipes which convey the separated fluid down into the separator liquid compartment.
For axial flow demisting cyclones it is a problem that particulate solids accumulate in horizontal sections within the cassettes themselves as well as in the horizontal oriented manifold. This may lead to blockage of the drainage pipes that completely or partly prevents the liquid from being drained and thus jeopardizes the function of the demisting equipment. This is of particular importance for separators to be operated at the sea bottom where there is no possibility of cleaning the drainage system.
Another problem associated with axial flow demisting cyclones where part of the gas is recycled, is that the liquid film formed at the bottom of the cassette may be sucked up and follow the recycle gas into the cyclone. Small deviations in the horizontal arrangement of the cyclones may lead to a “thick” layer of liquid in the corner furthest away from the drainage conduit. Particularly cyclones with a horizontal arrangement may be vulnerable to this negative effect of inclined installation because the horizontal distance between drainage conduit and the corners of the cassette is especially large. For a standard embodiment of a cyclone cassette the liquid film will be quite close to the cyclone tube and the risk of entrainment of liquid will be present because the liquid and gas impulse out of the cyclone tube will penetrate (into) the liquid layer.